Environmental Factors

Introduction

In previous units we have discussed about the earth’s environment its components and human activities. Now we know that the earth’s environment is made up of abiotic and biotic components, thus constituting living and non-living environment respectively. In this unit we will learn about various factors of environment such as temperature, light, water, humidity, weather conditions, fire, currents, pressures etc. and their effects on the living organisms. Apart from this we will also discuss about limiting factor that have detrimental effect on the organisms at their maximum and minimum levels. With the knowledge of these factors viz., physical and limiting it is easy to understand the working of various interlinked systems of environment.

Any constituent or state of the environment which have direct or indirect influence on the distribution, growth, development, reproduction, behaviour and survival of an organism in any specific way is called environmental factors or ecological factor or eco factor.

Each abiotic components of environment are a complex of various factors. For example, atmosphere includes factors light, temperature, moisture, gases. Hydrosphere includes water and lithosphere includes soil and minerals. These factors can also be categorized as: direct, indirect and remote.

The direct environmental factors are those which influence an organism directly such as light, temperature, atmosphere, humidity, soil, water and soil nutrients. On the other hand, indirect factors express their impact through direct factors such as wind, precipitation, soil structure. The remote environmental factors express their effect remotely through indirect factors which in turn influence the direct factors. The best examples of this are topographic factors like altitude. These environment factors are highly diversified, intricately mixed, inter linked and interrelated with one another and show variation from place to place.

Therefore, different types of organisms whether plants, animals or microorganisms they are able to survive and flourish under different environmental conditions. Due to morphological, physiological and genetic modifications the organisms are able to adjust and adapt according to the surrounding environmental conditions. This is called epharmony Those factors of the environment which determine the distribution of an organisms on the planet earth are called determining factors.

They are divided into the following five groups:

Climatic or aerial factors- Physical forces and material factors related to aerial environment. These include-light, temperature, rainfall (precipitation), humidity etc.
Topographic or physiologic factors- These are related to surface behavior or physical geography of the earth e.g., steepness of slopes, direction of mountain chains, altitude.
Edaphic factors- Related to both physical and biotic factors of the soil such as the formation and composition of soil.
Biotic factors- It includes all the types of interaction and relations among the living organisms- plants, animals and micro organisms.
Fire-It is the physical factor generally the outcome of anthropogenic activities.

All the above-mentioned factors except biotic factors constitute abiotic or physical factors. These factors can be well understood and explained with the model of a Plant, where edaphic factors correspond to root while climatic and biotic factors correspond to shoot (Fig.1).

Interaction of Environmental Factors

All the factors of the environment (physical and biological) interact either positively or negatively with each other. A change in one factor causes a change in another factor(s). Similarly change in any physical factor brings about a change in biotic factors or component and vice- versa. An organism under natural environmental conditions is influenced by many factors at the same time. Thus, all environmental factors function in conjunction not in isolation.

Broadly, we can say that all the life forms on this earth are influenced by numerous diversified environmental factors and conditions, where the effect of one factor is modified by one or many factors. Therefore, it becomes customary to study the effect of each environmental factor separately (analytical approach). But to have a broad holistic approach it becomes mandatory to study the factors taking into account all the possible parameters to draw conclusion related to the impact of these factors on the living organisms as changes in the earth’s environment are responsible for the evolution of life forms on this planet Earth.

Physical Factors of the Environment

The physical factors are mostly concerned with their effect on the living organisms. The important physical factors of the environment are as follows (Fig.2):

Light
Temperature
Water
Wind
Humidity
Weather
Soil
Current and Pressure
Fire

Light

It is one of the most essential ecological and physiological factors, without which existence of life on the earth cannot be imagined. Sun is the main unidirectional source of light and energy and has great ecological significance. Light varies in wavelength, intensity and duration (length of the Day). Apart from this it also acts as limiting factor at its maximum and minimum levels. The various effects of light on various living entity and their functioning can be summarized in following headings:

(A) Effects of light on Vegetation: Light is required by plants to perform the process of photosynthesis, in which solar energy is converted to chemical energy. Plants directly rely on light to prepare their food therefore; they are correctly said producers. Plants are categorized into Short Day Plants (SDP) and Long Day Plants (LDP) on the basis of duration of light required for their growth, development, reproduction (Flowering). A general thumb rule is that 1% increase in the duration of available light period will result in 1% plant growth and ultimately 1% increase in Plant yield. On the basis of their tolerance to light intensities, they are categorized into following groups:

Sciophytes- plants that prefer shady places and have sparsely distributed thin and larger leaves. Heliophytes- plants that prefer to grow well in direct sunlight and have crowded small and thicker leaves.Thus, the effect of light on plant can be summarized as Follow (Table - 1)

Table 1. Light and its effect on Plants.

Features	Effects
Chlorophyll	Needed for the formation of chlorophyll. Determines the number and position of chloroplasts.
Transpiration rate	Light regulates the opening and closing of stomata thus affecting the rate of transpiration
Distribution	It also affects the distribution of plants on earth surface. Due to less light intensity in polar regions plants distribution is highly restricted in comparison to tropical and equatorial regions.
Pigment	High intensity of light results in formation anthocyanin pigment responsible for floral colours. Carotenoids and flavin mediate photo response. Phytochrome pigment that have important role in seed germination require red light for its formation.
Orientation	Plants show phototropism. Stem show positive phototropism (towards light) and root of the plants show negative phototropism (away from the light).
Photosynthesis	Blue and red light are useful for photosynthesis
Germination	Light also affects seed germination either by promoting or inhibiting it especially, red and blue light promote seed germination.
Development	Light intensity has tremendous effect on the development of vegetative (leaf, root and stem) and reproductive (flowers, fruits and seeds) structures. Diffused or low intensity light favours development of vegetative structures and intense or high intensity light favours the development of reproductive structures. Very low or less than optimum light retards the plant growth.
Other specialised reactions	Opening and closing of flowers, folding and unfolding of leaves of some plants is also governed by change in light intensity. Example, Portulacca commonly known as office glory or morning flower opens up when sun rises and closes with the sunset.

(B) Effect of light on animals: As an important ecological factor, light has pronounced affect in animals too. It affects the distribution, growth, development, reproduction, locomotion etc. Light also regulates the production and release of certain hormones in animals. The effect of light on animals can be explained in following points given in Table - 2.

Table 2. Light and its effect on the Animals.

Features	Effects
Distribution, Colour and Structure	Light influences the distribution, colour, morphological structures, and formation of pigments responsible for colouration in animals. In cave animals’ colour and eyes are lacking or greatly reduced. In case of other terrestrial animals’ colouration is prominent and for this purpose have photoreceptors. Aquatic animals living at the depth of water bodies where penetration of light is not possible are either blind or have reduced or telescopic eyes.
Development	Some organisms are able to show normal development only in the presence of proper sunlight else their development is retarded and even result in mortality. E.g., Salmon fish. But on the contrary solmon and trout eggs are killed when exposed to direct sunlight. Mytilus larvae prefer darkness during early stage of development.
Locomotion and Orientation	There are three types of locomotion based on light: phototaxis (towards or away from light), photokinesis (movement on the basis of light wavelength and phototropism (movement of a particular part of the body). An increase and decrease in light intensity affect the orientation behaviour in animals. For example, Planaria, a platyheminthes, earthworm an annelida prefers dark area and shows movement away from light as light intensity increases there is increase in the rate of change of direction (negative phototaxis). Euglena a protozoan show positive phototaxis. Example of photokinesis - Locusts stops their flight when sun is hidden by clouds. Dendrocoelum lacteum (flatworm) that prefer dark, wet damp places when exposed to dim light turns occasionally but the rate of turning increases as the light intensity increased. Similarly, ammocoet larvae of Lamprey Lampetra planeri is usually found buried with their head pointed in downward direction in the bottom of water bodies when exposed to high intensity light show active swimming that helps to burrow them again moving away from light source. Similarly, sunlight play significant role in honeybees. They are able to orient toward food source and home only when they use sun as a compass to find out the direction of food source. Some insects like moths and other nocturnal animals like bats, owls etc., are active only when there is dark or when very low intensity of light is present.
Metabolism	Metabolic activities are also affected by fluctuation in light intensity. Increase in light intensity increases food intake and enzyme activities in most of the organisms.
Biological clocks	Daily light cycles (Day and Night) have profound role in the development, metabolic activities and behaviour of an organisms. They help in determining daily sleep hours and activity period of an organism. For example, humans and most of the animals are active during the day and their activity ceases during night (also known as Biological Clock). Copepods and zooplanktons come to surface of water as the night advance and with the onset of day again return to bottom.
Reproduction	Light have significant effect on the reproductive activities of many organisms. Duration (day length) and intensity (low or high) of light is decisive in initiation and inhibition of breeding. Gonads in birds become active only during summer when light intensity is high. In case of catfishes increased day length is responsible for attainment of maturity. Quail are long day breeder. Sheeps are short day breeder.
Migration	Migration in most migratory birds, eels and salmon fishes are affected by photoperiodism. Some birds migrate towards north in summer when days are long and migrate towards south in response to short days of winter.
Pigment formation	The light intensity also changes colour of skin. Pigment formation is induced by light e.g., skin colour of lizards and frog becomes light in bright light. In humans due to exposure to direct sunlight for long period results in tanning and even darkening of skin due to the production of melanin.

Temperature

The temperature is the most familiar and important physical factors of the environment and exerts its influence upon living organisms in several ways. The body temperature of many animals fluctuates with the change in the environment temperature. Such animals are called cold-blooded or poikilothermic (ectothermic) animals. e.g., all animals like reptiles, fishes, amphibians except birds and mammals. On the other hand, birds and mammals are able to regulate their body temperature at a constant level irrespective of the surrounding temperature and are known as warm-blooded or homeothermic (homoiothermic) or endothermic animals. In comparison to above mentioned two categories some animals have limited power of temperature regulation and they are called heterotherms, e.g., the monotremes and some marsupials. In all cases the temperature of an animal depends on the balance of those factors which tend to add heat and those which tend to decrease it.

Temperature regulates all biological, physiological and chemical processes. It is also responsible for the zonation and thermal stratification that occur in both water and land environment. It plays significant role in the distribution, growth, development, respiration, reproduction, survival etc. Temperature varies greatly from place to place in different environment such as aerial, terrestrial, aquatic (fresh or marine). Temperature variation is much more pronounced in terrestrial environment in comparison to aquatic. Not only this, there is fluctuation in day and night temperature. Even there is great variation in temperature of different regions of earth. Temperature of polar region is very cold and of tropical and equatorial region is hot. The effects of temperature arenumerous and pronounced which can be discussed under the following headings.

a) Effect of temperature on distribution: The temperature act as an essential limiting factor in the distribution of plants and animals. Temperature can limit the distribution of any species at any stage of the life. The temperature particularly affects the distribution of poikilotherms or the so-called cold-blooded animals whose body temperature is mostly dependent on that of the environment. The temperature is not a limiting factor for most birds and mammals and this is the reason why only the birds and mammals occur in the coldest as well as in the warmest parts of the world. For instance, coral reefs require a minimum temperature of 21oC for their existence;

therefore, they don’t exist in colder regions with temperature below 21oC. Aquatic animals have a narrow range of tolerance to temperature than terrestrial animals.

Temperature also plays significant role in the distribution of plants. Latitude and altitude of a place also brings variation in temperature. These three together are responsible for the type of vegetations of a particular geographical area. For instance, vegetations of tropical and subtropical regions are different from that which is found in temperate and alpine regions. Earth’s vegetation can be chiefly divided into following four categories on the basis of temperature prevailing in a particular geographical area:

Microtherms- Plants that can survive Inlow temperature e.g., plants of temperate and high-altitude regions dominated by mixed coniferous type forest.
Mesotherms- Plants that can tolerate high temperature of summer and low temperature of winters e.g., deciduous forests of tropical and subtropical regions.
Megatherms- Plants those are capable ofsurviving in high temperature prevailing throughout the year e.g., plants of tropical rain forest.
Hekistotherms- Plants that can tolerate extremely low temperature e.g., plants growing in alpine regions.

b) Effect of temperature on metabolism: It regulates the metabolic, enzymatic, hormonal activities both in plants and animals. The rate of metabolism and enzyme activities increases with increasing temperature. Not only this, temperature also influences the rate of transpiration, evaporation, photosynthesis and respiration.

c) Effect of temperature on growth, development and reproduction: An optimum temperature is necessary for the process of growth and development to proceed at a normal rate and it varies from species to species. In oysters change in optimum temperature results in reduced growth.An extreme of temperature act as limiting and even proves fatal. Temperature also determines the breeding seasons of organisms. Some animals breed throughout the year and some in winters or summer or spring depending on the temperature. Temperature affects the fecundity (egg laying capacity or ability to produce young ones)and sex-ratio in animals. For example, Daphnia (a small planktonic crustacean)commonly known as water flea reproduces parthenogenetically resulting in emergenceof females from the eggs. But when the temperature of the environment is increased, they reproduce sexually giving riseeither male or female individuals. In case of alligators and crocodile’s temperature determines the sex. Only male offspring’s hatches from the eggs when temperature is around 340C or above and when temperature is around 300C all female offspring emerges from the eggs. In insect’s optimum temperature favours rapid growth and multiplication whereas extremes temperature slows down this process or have adverse effects. Structures like spores, cysts, pupa, cocoon, seeds are produced to cope up with unfavourable environment conditions (like extremes of temperature). Plants undergo dormant or inactive state known as Dormancy during which plant stops growing actively, also known as resting period. Plants also show seed dormancy (seed germination is ceased) to avoid unfavourable conditions. Some animals undergo hibernation (winter sleep), a state of inactivity during winter to conserve energy and some aestivation (summer sleep), a state of inactivity during summer. Insects undergo diapause, aperiod of suspended development.

d) Temperature and moisture: The interaction of temperature and moisture depends on the relative as well as the absolute values of each factor. The temperature exerts a more severe limiting effect on organisms when the moisture is either very high or very low. The moisture plays more critical role in the extremes of temperature. For example, the cotton boll weevil cannot develop if the relative humidity is less than 40% or more than 88%, no matter how favourable the temperature may be. The animal remains dormant, regardless of the humidity, if the temperature is lower than 10oC or higher than 39oC.

e) Effect of Temperature on animal behaviour and structural modification: Certain animals show variations in responses to certain stimulus with variation in temperature. It may show positive response to one stimulus at one temperature and negative response to the same stimulus at a different temperature. Temperature changes are also known to affect structural changes. Drosophila may undergo structural modifications at high temperatures. Facets of eyes, eye size and change in number of eggs than usual i.e., 6 is also affected by change in temperature. This can be well explained by the phenomenon of cyclomorphosis, which explains the relation between seasonal changes in temperature and change in body structure accordingly.

Thus, cyclomorphosis can be defined as different structural modifications undergone by certain animals to cope up with the seasonal changes in temperature. It is observed in head size during different seasons in Cladocera (Daphnia). Colour patterns in many insects can be induced or changed by regulating the temperature under which they develop.

Water

Water serves as a fundamental media for existence of life forms on earth due to its certain unique properties. It is also referred as universal solvents. It effects growth and geographical distribution of both plants and animals. A plant varies in the amount of water they are able to absorb from the soil and transpire. Plants that have more water requirement and have high transpiration rate are limited to habitat where there is sufficient water. Water act as the limiting factor in land environments than aquatic as the amount is subjected to great fluctuations. Aquatic animals also maintain a proper balance of water by the process of osmoregulation. Water also plays significant role in metabolism. The heat holding capacity of water helps in the maintaining constant temperature.On the basis of amount of water required for excretion animals are divided into three main categories: ammonotelic (more water is required for elimination of waste), ureotelic (less water is required for elimination of waste) and uricotelic (least amount of water is required for elimination of waste). If the total gain in water is equal to loss, the animal is said to be in water balance. On the other hand, if water loss is not covered by an equal gain, the animal is called in negative water balance.

Humidity

The moisture in air in the form of water vapour is called humidity. In other words, humidity represents the amount of water vapour in the atmosphere. Humidity along with light and temperature plays an important role in regulating the activities and the distribution of organisms. Animals in rain forests live only where the air is almost saturated with moisture`. Desert animals live where the air is extremely dry. According to Gloger’s rule, the birds and mammals of warm humid regions tend to be darker in colour than inhabiting the cold or dry regions of their geographical range. Insect Lepisma saccharina (silver fish) reproduces only when relative humidity is 85 to 90 percent. Young ones die when relative humidity is less than 70 percent. Tse -tse fly vector of Trypanosoma gambiense do not survive when relative humidity is more than 88 percent. Larva of Silkworm, Bombyx mori do not undergo pupation when air is moist. Humid conditions also Favour the growth of several microorganisms and fungi.

Wind

Wind has direct(mechanical) and indirect (Physical) effect on living organisms. This effect is more pronounced in case of plants. Mechanical effects of high speed of wind include bending, flattening, and breakage of tree branches and uprooting of trees. Physical effects include desiccation, salt spray near seacoast, dwarfing and deformation. Not only this, wind also had positive aspect that is it helps in dispersal of pollens, seeds, small organisms, and wind pollination (anemophily). It is also essential for movement of clouds resulting in rain in various parts of the world.

Weather

Many of the physical factors like the temperature and light are correlated with seasonal changes in weather. Some animals in the temperature zones can tolerate a wide range of temperature fluctuations. Many, however, are killed at extremely low temperature. But their races are preserved by their spores, eggs and larval forms, which can withstand such extreme conditions. The hazards of winter are met by animals in various ways like migration, hibernation, and changes of food habits. This is the reason why food chains in winter differ greatly from those in summer.

Soil

Soilis an edaphic factor of great ecological significance which is formed by the breaking down and decomposition of rocks due to weathering (action of rain, water, wind, temperature etc.), the action of soil organisms, earthworms, and interactions of various chemical substances. It determines the distribution of plants and its type directly or exerts indirect effect with the help of other essential factors like temperature and water. It provides minerals, nutrients and medium for support and anchorage to plants. It also provides homes for large number of organisms like bacteria, fungi, protozoa, nematodes, rotifers, earthworms, molluscs and burrowing animals. The importance of soil may be realised from the statement of Sundar Lal Bahuguna (1987): “the eternal truth that soil and water are the two basic capitals of humankind and natural forests are the mothers of rivers and the factories for manufacturing soil”.

Currents and Pressures

Currents are generally movement of large quantity of water from one place to another. Water currents have profound impact on the distribution of various organisms particularly planktons. They also redistribute heat, nutrients, gases, minerals, food particles and other substances supporting many aquatic organisms and ultimately, ecosystems of the world. Oceanic currents regulate the global climate.

In the ocean hydrostatic pressure plays significant role. It increases proportionately with increasing depth of the water body. The impact of pressure in depth of the sea is more on animals which have air spaces, cavities, bladders or sacs in their body. Therefore, to overcome this stress organisms living in deep sea water either don’t have air spaces, cavities or bladder to avoid crushing by pressure and in any case, they are present, then they are filled with fluid or oil. Thus, oceanic pressure has depressing effect slowing down the pace of life.But this is quiet opposite in case of land. Here the atmospheric pressure decreases proportionately with the increase in altitude above sea level.

Fire

Fire is both limiting and regulatory ecological factor. It is caused by physicals well as biological phenomena. Under physical causes comes lightening, sun heating during summer, rubbing of trees and volcanic eruptions. Biological causes are further two types: natural and anthropogenic. Fire scars provide us with valuable information regarding the history of a forest. The fire used for clearing the forest has several known impacts:

Destruction of flora and fauna
Destruction of organic matter (Humus)
Climate change
Disturbances of Biogeochemical cycles
Removal of Fire sensitive species thus disturbing ecological balance.

Apart from above mention impacts of fire it has certain ecological importance.

Regeneration of vegetation by stimulation of dormant seeds lying down in soil.
Increases porosity of soil by increasing noncapillary pores in the soil.
Many pests and parasites are killed by fire.
Used for removal of weeds
Maintenance of soil pH.
Maintenance of grasslands
Destruction of litter by burning.

Limiting Factors

Existence and survival of an organisms or a group of organisms depends upon prevailing complex environmental conditions. Any condition that approaches or exceeds the limits of tolerance is said to be a limiting condition or a limiting factor (Odum, 1983). Under steady state conditions an organism needs essential materials or substances that are necessary for survival, growth and reproduction. Therefore, any factor that tends to slow down this process is known as limiting factor. This can be well understood with the help of following laws that will give us an insight into how organisms are greatly influenced by their environment?

Liebig’s law of minimum

An organism proves to be weak when essential elements needed for its growth and development is not available in optimum quantity. Justus Liebig in 1840 explained clearly in his studies on the effect of various environment factors on the growth of plant and he reached to the conclusion that the crop yield was frequently limited not by the essential elements like carbon dioxide and water needed in large quantities that was available in abundance, but by the other elements like zinc that was needed in minute quantities and was scarcely available in the soil. This was later known as the “Liebig’s law of the minimum. "According to this law “growth of a plant is dependent on the amount of food stuff available to it in minimum quantity”. In a simplified form growth is dictated not by total available resource but by the resource that is scarce or absent.

Liebig-Blackman law of limiting factors: The Liebig law generally took into account chemical material required by plants in their natural environment. The first constraint associated with Liebig law was that it was strictly applicable only under steady state condition. That is, when there is balance between energy and material inflow and outflow. Therefore, the statement given by Liebig was combined with the law of limiting factors first proposed by British physiologists F. F. Blackman (1905) who studied the factors affecting the rate of photosynthesis and observed that the rate of photosynthesis in the plant was mostly governed by the level of the factors that was operating at a limiting intensity. According to him amount of carbon dioxide, water, sunlight, chlorophyll and temperature of the chloroplast are the five factors that have decisive role in controlling the rate of photosynthesis. The two-concept combined together form the so-called Liebig-Blackman law of limiting factors.

The factor interaction: The second important consideration is factor interaction. In this case high concentration or availability of some substances or the action of some factors other than that factor which is present in minimum quantity can also modify or manipulate (increase or decrease) the rate of utilisation of the factor that is present in minimum. In some cases, the organisms are even capable of replacing the deficient factor or the substances of the environment with a chemically closely related substance. This can be explained with the help of calcium level in molluscs. Strontium has physical and chemical properties similar to calcium and barium and occurs naturally in the form of celestine and strontianite. Molluscs to some extent substitute calcium with strontium in their shells as the latter is available in abundance in their environment. Similarly, plants growing in shady area require zinc in low quantity as compared to those growing in direct sunlight. Therefore, low concentration of zinc will act as limiting factors in case of plants exposed to direct sunlight.

The law of minimum was further restated or modified broadly by Taylor in year 1934.According to him "The functioning of an organism is controlled or limited by that essential environmental factor or combination of factors present in the least favorable amount. The factors may not be continuously effective but only at some critical period during the year or perhaps only during some critical year in a climatic cycle.”But according to Odum, the concept of minimum should be restricted to chemical materials viz., oxygen, phosphorus etc., which are essential for proper physiological growth and reproduction of an organism. While other factors and the limiting effect of maximum are included in the law of tolerance. Thus, Liebig law of minimum deals with one aspect of the concept of limiting factors.

Shelford law of tolerance

Not only too little of something act as the limiting factor as proposed by Liebig but also too much of a substance as in the case of heat, light and water also have limiting effect. Thus, an organism has an ecological minimum and maximum in between these two zones there is range known as limits of tolerance (fig.3).

Figure 3: Graphical representation of maximum, optimum and minimum limit with respect to environmental factors (Like Temperature).

The concept of maximum and minimum in the survival of an organism was incorporated in the law of tolerance by V. E Shelford in 1913.According to this law the abundance or distribution of an organism can be controlled by certain environmental factors where levels of these exceed the maximum or minimum limits of tolerance of the organisms. Any value lying between these critical limits will naturally fall in the limits of tolerance(fig.4) for an organism. But before this range is crossed on either side and limits of tolerance are reached, there is a zone of physiological stress here the organisms try to adapt itself. Beyond this zone, if the organism is exposed for a considerable time it will lead to the disappearance of the species from that area as long as such conditions prevail.

Fig.4. Graphic illustration of Law of tolerance

Further according to this law there are following two zones for every environment factor.

Zone of tolerance
Zone of intolerance

1) Zone of tolerance - It lies between the lower and upper limit of tolerance. It is the most favorable zone for the growth, development and survival of an organism. Therefore, it is also known as zone of compatibility or the biokinetic zone or the zone of capacity adaptation.

Further this can be subdivided into:

Optimum zone- with most favourable and advantageous environmental conditions for the growth and development of an organisms and result in population rise or build-up.
Critical minimum zone- lowest bearable limit below which activities of an organisms inhibited so that the organism could survive in adverse environmental conditions
Critical maximum zone- highest maximum bearable limit beyond which the activities of an organism is ceased to survive adverse environmental conditions.

2) Zone of intolerance -The zone at the either end of the zone of tolerance is known as zone of intolerance or lethal or resistance zone. Here in this zone the environmental conditions are least favorable. The organisms in the zone are not able to withstand harsh climatic condition as a result unable to survive. Therefore, in this zone organisms are absent.

Some subsidiary principles to the law of tolerance are mentioned below:

Organisms have a wide range of tolerance for one environmental factor and a limited range for another.
Only those organisms are widely distributed which have wide range of tolerance for all the environmental factors.
When one environment factor is not conducive for an organism than the limit of tolerance for other factors is also greatly reduced. For example, when the availability of nitrogen in the soil is less than the optimum the resistance of the grass to the drought is reduced. In other words when the amount of nitrogen is low in the soil than more water is required to prevent plants from wilting (Penman, 1956).
In nature organisms are not always provided with optimum range of a particular physical environmental factor. Instead other environmental factor exerts significant influence on the organism. For example, certain tropical orchids grow better in full sunlight than in shade when they are kept cool (Went, 1957). But in nature they grow only in shade as they cannot tolerate the heat of direct sunlight. Even interspecific interactions like competition, predation, and parasitism also prevent organisms to take benefit of optimum physical environmental conditions.
Physical environmental factors prove more limiting to various reproductive stages (seeds, eggs, larvae) of plants and animals due to very restricted range of tolerance. For example, Adult blue crab (marine animal) can survive in sea as well as in fresh water with high chloride content. But on the contrary their larval forms are unable to survive and reproduce in fresh water. Here in this case the river water acts as limiting factor. Similarly, Cypress tree can grow submerged in water and even in dry upland, but needs moist unflooded area, a combination of both the conditions for the growth and establishment of seedling. Species having narrow range of tolerance are represented by prefixes “steno” meaning narrow and those having wide range of tolerance are represented by “Eury” meaning wide (Fig.5). Therefore, following terms are used to show relative degree of

Fig.5. Tolerance curves showing two main types of responses of an organism to different environmental factors

Eurythermal- organisms which are able to tolerate a wide range of temperature.
Stenothermal- organisms which are able to tolerate small range of temperature.
Stenohaline- able to tolerate only a narrow range of salinity.
Euryhaline- able to tolerate wide range of salinity.
Stenophagic- has a narrow range of food.
Euryphagic- has a wide range of food.
Stenohydric- tolerant of a narrow range of moisture or humidity.
Euryhydric-tolerant of a wide range of moisture or humidity.
Stenobathic- can tolerate only limited changes in depth.
Eurybathic- tolerant to both deep and shallow water.
Stenoecious- have restricted range of habitat or niche.
Euryoecious-have broad range of habitat or niche.

Importance of limiting Factors

It is valuable because it gives the environmentalist an insight of complex environmental conditions.
To discover factors those are operationally significant (Odum, 1983).
To determine how environmental factors, affect an individual, population and community.

Case Study
Great South Bay on Long Island, New York is the best example of concept of limiting factors. How too much of one factor act as limiting for one organism and proves favorable for other, thus changing the ecosystem of that area. Here in this case, the duck manure from the large duck farm situated in this Island acted as the main cause for the addition of excess amount of nutrients into the water.
This condition resulted in the increase in the population of phytoplanktons, that resulted in low nitrogen and phosphorous ratio which acted as the limiting factor and there was shift in the type of producers i.e., diatoms, green flagellates and dinoflagellates were replaced by small green flagellates of genera Nannochloris and Stichococcus. This shift in the producer created problem for blue point oyster (that supported profitable industry of the bay) thriving on original phytoplankton populations. The oyster population was unable to utilise small green flagellates as food and gradually disappeared from that place. A major setback for the oyster industry of Great South Bay. Not only this, other shellfish were also eliminated from that place. The reason for their elimination was that the green flagellates (new type of producers) grew well when nitrogen was available in the form of urea, uric acid and ammonia which in this case provided by duck manure. Whereas the diatoms, original inhabitants of the bay required inorganic nitrogen (nitrate) which is a long process.
So, this case study is a very excellent example of how a species exclusive to particular area or region is replaced by another rare species with change in any physical factor. This case study was very well known by “The Duck vs. The Oyster.” (Odum, 1983).

Summary

Environment of organisms plays very crucial role in the survival. Mostly the physical factors have significant effect on the living components. Living organisms tries to adjust and adapt themselves to different factors of the environment as they have different requirements and tolerance for different factors. The presence or absence of physical factor(s)decides or limit the actual distribution and abundance of plants and animals geographically. This stress in the organisms to cope up with complex environmental conditions has led to morphological and physiological modifications in organisms in due course of evolutions. Those whichare not able to do so are either limited to certain places or become extinct. Further, the physical factors have different impacts on an organism or population at different time, place and conditions. Environmental condition that exceeds the limits of tolerance of an organism act as a limiting factor. Apart from this, maximum and minimum of an environmental factor too have limiting effect. A range in between these two indicates tolerance level of an organism. The concept of limiting factors helps the environmentalist to know about environmental conditions that are critical and limiting. That is abundance and scarcity of essential nutrients, minerals, physical and chemical nature of the environment significantly decides the existence of organisms on this planet earth.

Thus, above knowledge can be utilized in increasing the production of crops in agriculture and managing pest population without disturbing the ecological balance.

Environmental Factors - Interaction, Physical Factors, Limiting Factors, Importance

Introduction